CRACK EDGE DIFFRACTION: A WAY TO UNDERSTAND HETEROGENEOUS RUPTURES AND THE SUPERSHEAR TRANSITION
DUNHAM,E.M. and CARLSON,J.M., University of California, Santa Barbara , CA 93106, email@example.com, firstname.lastname@example.org; and FAVREAU,P., Institut de Physique du Globe de Paris, 75252 Paris, France, email@example.com.
As a rupture advances and the fault begins to slip, shear traction breaks down over some length scale. This breakdown zone is constructed as a superposition of point shear stress drops, with the region ahead of the rupture front locked. Waves released by the stress drops overtake and diffract off of the rupture front, carrying changes in the stress field to the locked section of the fault. We explain the connection between Rayleigh and head waves on the fault and the allowed rupture velocities (sub-Rayleigh and supershear). The supershear transition emerges naturally from this picture, as head waves from the breakdown zone, travelling at the P-wave velocity, overtake the rupture front and diffract into shear waves. These carry the driving force that pushes cracks supershear. We then apply this theory to explain the dynamic growth of shear cracks through strength heterogeneities. As shown in numerical experiments by Dunham et al. (2003), a rupture encountering a high strength region encircles it before failure, a process that generates intense pulses of radiation. Sufficiently strong barriers trigger transient bursts of supershear propagation. Our approach explains how the curvature of the rupture front focuses the waves released during crack growth. The diffraction pattern created by these waves matches the observed shape of the rupture front and the ensemble of slip pulses trailing it.